1. Field of the Invention
The present invention relates to liquid crystal compound, liquid crystal composition and liquid crystal display element. More particularly, it relates to a novel liquid crystal compound having a perfluoropropenyl group (—CF═CFCF3) at the end, a liquid crystal composition containing the same and a liquid crystal display element containing the composition.
2. Related Art
According to its display system, liquid crystal display element is classified into several modes such as TN (twisted nematic), TN-TFT (twisted nematic-thin film transistor), bistable TN (bistable twisted nematic), STN (super twisted nematic), IPS (in-plane switching), GH (guest host), DS (dynamic scattering), VA (vertical alignment), OCB (optically compensated bend), ECB (electrically controlled birefringence) and PC (phase change). Physical property which is necessary for liquid crystal composition used for the element varies depending upon such modes.
Such an element contains a composition having an appropriate physical property. In order to improve the characteristic of the element, it is preferred that the composition has an appropriate physical property. The general physical property which is necessary for a compound which is a component of the element is as follows. (1) Chemical stability and physical stability. (2) A high clearing point. Clearing point is a transition temperature from a liquid crystal phase to an isotropic phase. (3) A low lower limit temperature for a liquid crystal phase. The liquid crystal phase means a nematic phase, a smectic phase and so forth. (4) Low viscosity. (5) Appropriate optical anisotropy. (6) High dielectric anisotropy. Compounds having a high dielectric anisotropy often have a high viscosity. (7) High specific resistance.
A composition is prepared by mixing of many compounds. Therefore, it is preferred that a compound is well miscible with other compounds. Since an element is sometimes used at the temperature of lower than an ice point, a compound having a good phase solubility at low temperature is preferred. Compounds having a high clear point or having a low lower limit temperature for liquid crystal phase contribute in a broad temperature range of a nematic phase in a composition. Preferred compounds have a low viscosity and an optical anisotropy suitable for a mode of the element. High electric anisotropy of the compound contributes in a low threshold voltage of the composition. Such a composition is able to give an element having the characteristics such as that usable temperature range is broad, response time is short, contrast ratio is big, driving voltage is low, power consumption is small, voltage retention rate is big and so forth.

The above compounds have low viscosities. Compounds having viscosities of the same level as those compounds and having higher dielectric anisotropy shorten the response time of the element and, therefore, they are particularly useful.
Compounds which have been known already are disclosed in the following documents. Mol. Cryst. Liq. Cryst., 191, 205 (1990), WO 9734855 A1 (EP 0 891 314 B1), WO 9221734 A1 (U.S. Pat. No. 5,403,512), and GB 19528085 A1 (U.S. Pat. No. 6,159,393).